What do you mean by “pure white”. All the templates start out as “white sheets of paper”. Just turn off the grid and you have a white background. Does that help? Not sure why you would think that the templates are not “white”

You can start with any blank white template, and change the paper size in Document Options to A1. If you use it often, save it as a template. Or start with an A1 template and erase all the titleblock text and lines you don’t need.

Note too that the grid that is visible on many of the templates can easily be turned off, and it won’t export or print anyway.